Silver halide color photographic materials, particularly those used in picture taking, are required to have specific properties such as high sensitivity and high quality image, especially good granularity. The demand for meeting these requirements is ever increasing today since the tendency toward using smaller cameras necessitates the reduction in the size of images such as in negative films. While many improvements have been made in technology of manufacturing silver halide color photographic materials and products with improved performance have been developed, most of them are unable to solve the problem of trade-off between the sensitivity and image quality. As is well known in the art, tremendous difficulty is involved in achieving a significant increase in the sensitivity of silver halide photographic materials, and there is no technology available today that is capable of increasing the sensitivity without impairing the resulting image quality (granularity, among other things). A particularly great difficulty is encountered in an attempt at increasing the sensitivity of a lower layer in a multi-layered silver halide color photographic material, i.e., a silver halide light-sensitive layer closer to the support, as well as at improving the quality (e.g. tone gradation) of the image formed in that layer, and no technology proposed to date has been found completely satisfactory. There are several factors that explain this difficulty: (1) "interlayer effect" or delayed diffusion of an aromatic primary amine developing agent into a light-sensitive layer closer to the support of a silver halide multi-layered color photographic material under development; (2) diffusion of a development retarding compound incorporated in that particular light-sensitive layer or in any other layer; (3) optical loss due to a non-sensitive layer or emulsion layer positioned in the upper part of the photographic material during exposure.
Various methods are known for sensitizing silver halide emulsions, and they include: modifying the step of physical ripening by, for example, making coarser silver halide grains; chemical sensitization with noble metal, sulfur, selenium or reduction sensitizers; spectral sensitization; sensitization by addition of fine silver halide grains to a silver halide emulsion; sensitization by addition of a silver halide solvent to the silver halide emulsion; and sensitization using a two-equivalent coupler or any other coupler that is capable of rapid reaction with the oxidized product of a developing agent.
Various techniques have also been proposed for providing an improved image quality. Particularly active efforts are being made to commercialize the method of incorporating in a silver halide color photographic material a compound that releases a development retarding compound during development. Two typical examples of this DIR compound are DIR-couplers (development inhibitor releasing couplers) that not only form a color dye by reaction with the oxidized product of a color developing agent but also release a development inhibitor, and DIR-hydroquinones or DIR-substances that release a development inhibitor but which are substantially incapable of forming a color dye upon reaction with the oxidized product of a color developing agent; these latter compounds form a colorless compound by reaction with the oxidized product of the color developing agent.
The use of these DIR compounds is highly effective for the purpose of providing an image of improved quality, but if they are combined with one of the known sensitizing techniques shown above, two serious defects arise. Firstly, the sensitivity of an emulsion layer containing a DIR compound that releases a development inhibitor imagewise during development is decreased due to the "interimage effect" that is detrimental to the development of that layer. This problem is particularly significant if the diffusibility of the development inhibitor released from the DIR compound (e.g. DIR-hydroquinone, DIR-substance or DIR-coupler) is small. The second problem is the "interimage effect" wherein the development inhibitor released upon development diffuses into an adjacent overlying or underlying emulsion layer, or even into an emulsion layer that is sensitive to light of a different color, and in that layer the inhibitor exhibits its development inhibiting action in accordance with the imagewise distribution specific to the layer where said inhibitor is initially incorporated. This interimage effect becomes significant if the released development inhibitor is a halide ion or organic heterocyclic compound having high diffusibility.
In order to avoid these two deleterious effects, the DIR compounds should be used in limited amounts, but then this makes it impossible to achieve the necessary sensitization by combining a particular DIR compound with one of the known sensitizing techniques; furthermore, the improvement in the image quality is far from being satis- factory.
Methods have been proposed for eliminating the above mentioned defects of using DIR compounds. According to one proposal, a non-light sensitive colloidal layer is placed adjacent to an emulsion layer containing a DIR compound. This adjacent layer contains substantially non-developable or low-sensitivity silver halide grains and is capable of adsorbing a development inhibitor released from the DIR compound. This non-sensitive layer per se is effective in reducing or limiting the adverse effects of the development inhibitor released from each silver halide color emulsion layer, and if this layer is placed adjacent each emulsion layer, the sensitivity of that emulsion layer is not greatly reduced. However, other serious defects occur, such as increased fog and reduced storage stability. These are fatal to the commercial value of the final photographic product.
The long felt need for providing a silver halide photographic material having high sensitivity and good image quality (especially high granularity) is yet to be satisfied.